In the voice transmission system, such as the digital enhanced cordless telecommunications (DECT), it is common to have build-in echo canceller to eliminate the echo. Also for those hand free cellular phones, the distance changes between the talker and the phone result in the inconsistent voice volume. To control the volume within the preset values, the automatic gain controller is generally introduced in the voice transmission system for processing the volume control.
Please refer to FIG. 1, which is a block diagram illustrating the circuit of the common voice transmission system. It includes an analog-to-digital converter 11 (ADC), a digital-to-analog converter 18 (DAC), a finite impulse response filter 15 and an automatic gain controller 13 (AGC). Digital signal converted from input analog signals through analog-to-digital converter 11 is multiplied by the first gain 12 and then minus the echo value calculated by the finite impulse response filter 15 so that the signal-energy is obtained. This signal-energy that has been processed by automatic gain controller 13 will be sent to loudspeaker 14. The phenomenon that the phone receiver has inconsistent voice volume could be prevented by such procedure described above. At the transmitter side, the signal received from the microphone 16 is multiplied by the second gain 17 and then is converted into an analog output signal by digital-to-analog converter 18.
Please refer to FIGS. 2a and 2b, which show the waveforms of the signals before and after treated by the automatic gain controller used in common voice transmission system. FIG. 2a shows the waveform at point A in FIG. 1 while FIG. 2b shows the waveform at point B in FIG. 1. As shown in FIG. 2a and FIG. 2b, two levels, an assigned level and a noise level, will be set by the automatic gain controller. The assigned level is the predetermined volume. If the volume is greater than assigned level, it will be diminished by the automatic gain controller. On the contrary, if the volume is less than the assigned level, the volume will then be increased by the automatic gain controller and controlled within the range of the assigned level. For the noise level, the very small volume often considered to be the noise wouldn't be processed.
However, in the practical application, using the fixed-point method to calculate the first gain 12 and second gain 17 may introduce noises. Also the finite impulse response filter 15 might not be able to eliminate echo entirely, that leaves the noise signals with small energy. These noise signals will generate uncomfortable noise if the bit number is not enough for the fix-point calculation.
From the above description, it is known that how to develop a new controller and controlling method with the advantages of low production costs and having the function of noise control has become a major problem waited to be solved. In order to overcome the drawbacks in the prior art, an automatic gain controller and the controlling method thereof are provided. The particular design in the present invention not only solves the problem described above, but also uses a noise level to be one of the controls in the control flow chart of the automatic gain controller for controlling the noise. It only needs simple circuit elements to implement the automatic gain controller of the present invention, and therefore greatly decreases the production costs. Thus, the invention has the utility for the industry.